Electric heating appliance



Feb. 9, 1932. H. N. sHAw ELECTRIC HEATING APPLIANCE Filed OO'C. 14, 1951 V zr INVENTOR Patented Feb. 9,y 1932 i HAROLD N. SHAW, F ERIE, PENNSYLVANIA,

NIAGARA FALLsQI NEW YORK,

PATENT OFFICE ASSIGNOR '10 GLOBAR CORPORATION, OF

A CORPORATION OF NEW YORK ELECTRIC HEATING APPLIANCE Application led October 14, 1931, Serial No. 568,841, and in Switzerland September 30, 1925.`

This invention relates to Ielectric heating appliances, and more especially to contact terminals for electric heating appliances employing rigid non-metallic resistors as the heating elements. c,

The chiefl factor which has retarded the commercial development of silicon carbide heating elements is the dilliculty of making satisfactory electrical contact. The 'terminals which have hitherto been commonly employed have been rigidly attached to the element. Under these conditions the terminal becomes almost as hot as the resistor itself, since it is `practically an integral part of the heating element. Arcing frequently takes place with this type of terminal owing to differential expansion, and no common commercial metal or alloy will withstand arcing at these high temperatures, so that the contact soon becomes disintegrated. The diiculties from this source have caused very elaborate means of making contact,` such as enlarged ends of considerable length to decrease the contact temperature, cumbersome split ring terminals, spraying the ends of the element with precious metals, followed by wrapping the ends with wire and glazin to protect the metal contact, and other simllar means.

I have found that very satisfactory electrical contact can be made by merely pressing the contacting member against the sur face of the element, preferably at its extreme end, and maintaining the contact by resilient pressure. Such a contact can be maintained up to very high operating temperatures for the resistor', as for example, upto 1100o C. or even higher, and the electrical resistance of the circuit is not increased appreciably by the resilient nature of the contact, even with fairly-'heavy currents. The chief advantage of such a .contact is the great temperature drop between the resistor and the metal facing of the terminal. For example, actual measurement has shown that when the temperature of the end of the rod is approximately 850 C., the terminal temperature may be as W as from 170 to 235 C. With highly heat resistant metals and. alloys, such for example, as iron-chromium alloys containing from to 30% of chromium, this greatly reduced temperature reduces the effect of arcing and disintegration to a point Where satisfactory contact can be maintained over a lnuch more prolonged period of time. In addition, it has made possible-a terminal facing of metallic aluminum, and I have found that this soft low melting vmetal practically eliminates the arcing Which occurs spring-pressed terminals in butt-end contactv with the resistor rod have a further advantage in the ready removal or replacement of the resistor rod, which can be accomplished by simply spreading the spring-pressed contacts.

The resistor elements employed are preferably rods of reerystallizedeor self-bonded silicon carbide, having their end terminal portions impregnated with elemental silicon, whereby the specific electrical conductivity of the end portions is greater than that of the main body of the resistor rod. The end portions, therefore, operate at a considerably lower temperature than the body portion. Such resistors are commonly referred to as having cold ends. Resistors of this type are described in the application of Ferdinand Eichenberger, Serial N o. 615,- 933, filed January 30, 1923, this country .under the trade name of Globars.

The combination of a resistor rod having silicon-impregnated cold ends With contact terminals pressed into yielding engagement therewith is highly desirable where the heating elements are operated at high temperatures, such as of the order of magnitude of 11000 C., particularly with contact terminals silicon carbide and are sold in A faced with relatively soft low melting point metal, such as aluminum.

While the main body of the resistor may be heated'to a brilliant incandescence, the silicon-impregnated cold ends which are in engagement with the Contact terminals operate at a much lower temperature. The contact terminals of aluminum or similar relatively soft good heat conducting metal used with silicon carbide resistors having' siliconimpregnated cold ends form a particularly advantageous combination and permit the main body portions of the silicon carbide resistors to be operated at high incandescent temperatures while successfully overcoming the difliculties heretofore encountered in maintaining a satisfactory electrical contact with the ends of the resistor rods, particularly for devices, such as domestic heating appliances, which are subject to intermittent service.

In the drawings, which illustrate the preferred embodiments of my invention,

Figure 1 is a sectional view of a non-metallic resistor and its contact terminals;

Figure 2 is a detail section along the line II-II of Figure 1;

Figure 3 is a sectional view of a modilied form of terminal; and

Figures 4 and 5 are a plan view and a section, respectively, of a modification.

Referring to the illustrated embodiment of the invention, there is illustrated in Figure 1 an electric heating appliance, such as an electric room heater having a back plate 2 and side plates 3. A rigid, non-metallic resistor 4, preferably of recrystallized silicon carbide, such asa Globar is mounted by means of the spring-pressed Contact terminals 5. Each contact terminal comprises a spring 6, preferably of a heat-resisting iron-chromium alloy, which is mounted on the back plate 2 by a screw 7 passing through insulating bushings 8 and adapted to receive an electrical lead 9. The spring carries a facing or contact plate 10 which bears directly against an end of the resistor 4. As shown in the drawings, the contact terminal is preferably hollowed, as indicated at 11.

The spring-pressed contact terminals 5 have resilient butt-end engagement with the .ends of the silicon carbide resistor rod 4, and

as above pointed out, with contact terminals of this character, there is a great temperature drop between the silicon carbide resistor and 4the metal facingofthe terminal.

The facing or contact plates 10 are preferably made ofaluminum, for the reasons here-l inafter explained. In cases where aluminum ca nnotv be used because of eXcesslve temperatures, and where the contacts are not subject to repeated heating and cooling, other suitable metals maybe employed, such as, for' example, Monel metal, or the nickelchromium, thechromium-iron and the chromium-nickel-iron alloys which form relatively thin protective oxide films under heat.

In Figure 3 there is illustrated a modification in which an end of the resistor 20 is resiliently held by contact terminal formed by twin springs 21 and 22, which are faced with aluminum contact pieces 23 and 24.

In Figures 4 and 5 is illustrated a modiiication in which the yielding contact is maintained between the aluminum contacts and the ends of the resistor rod, by an arrangement consisting of a iiXed contact at one end of the rod and a resiliently held contact at the other end of the rod. The heating device as shown in Figures 4 and 5 comprises a chamber 30 formed of a bottom plate 31, end plates 32 and 33, and side plates 34 and 35. A protecting grid 36 is mounted over the top of the chamber.

Two resistors 37 of the Globar type are mounted in the chamber 30. These are of the usual cold-ended recrystallized silicon carbide Globar type. Each resistor is mounted between two contact terminals 38 and 39, respectively. Contact 38 is rigidly mounted on the end wall 33. It consists of a block 40 of aluminum having a recessed face 4l to engag-e the one end of the resistor 37. The block 40 has an extension 42 which extends through an insulating bushing 43 and which is provided with suitable clamping screws 44 for making electrical connections.

The other contact portion 39 comprises a spring 45, preferably of a chrome iron alloy, having a facing 46 of aluminum or other soft metal -which is integrally united with the backing 45, as by riveting. The terminal 39 is mounted on an insulating block 47 secured to the end wall 32. The two contact portions 39 arel connected together by a conductor 48 to complete the electrical circuit.

As will be readily apparent from Figures 4 and 5, the aluminum contact terminals 38 and 39 are held in yielding engagement with the ends of the resistor element 37 ,y because the spring-pressed terminal 39 resiliently presses against the resistor 37, which in turn is resiliently pressed against therigid contact 38. So far as securing the yielding or resilient engagement between the aluminum Contact terminals and the ends of the rod, the arrangement shown in Figures 4 and 5 in which one terminal is iiXed while the other is spring-pressed, is the equivalent of the arrangement shown in Figure 1 in which both terminals are spring-mounted, and therefore when I speak of having the contacting members heldin yielding engagement with the terminal portions of the resistor, I intend to include either of these two arrangements or any other arrangement whereby resilient engagement between the contacting members and the resistor is secured.

Even though metallic aluminum mightcycles.

upon first consideration be rejected as entirely unsuited for the purpose, I have found upon actual test that in continuous-intermittent service, contacts of metallic aluminum between the heating element and the metallic spring clips last'alniost indefinitely in service where the contact members are operated at temperatures at or below the melting point of aluminum. The continuousinterinittent service referred to consisted in subjecting the heating device to a number of temperature cycles by repeatedly heating theA resistors to about 850o C. by applying electrical energy for a period of fifteen minutes and then allowing them to cool for another period of fifteen minutes.

These tests, which represent the type of service required in domestic heating devices, namely, intermittent service 'involving a large number of temperature cycles, because of the fact .that the device frequently is used for a few minutes and then turned off only to be used again after a short interval, are particularly important in the development of. a satisfactory heating device of this nature, inasmuch as the excessive heating which destroys the contact surfaces is produced by a number of factors, including arcing in addition to resistance heating.

Arcing is much more pronounced with 1ntermittent service than with continuous service because of mechanical movement due to expansion. A terminal which will last continuously for more than a month will often burn out in two or three days when tested intermittently.

I have found that aluminum-faced resilient contacts will withstand more than 2000 temperature cycles in which the cycle consists in heating the resistor to about 850 C. for fifteen minutes and then allowing it to cool for fifteen minutes. The hard metalcontacts made from the alloys usually employed for heat-resisting purposes become electrically insulating during such a test, usually after a comparatively small number of Iron and nickel base alloys which melt at temperatures of approximately 1400o C. are melted locally and badly oxidized during the intermittent test becauseof the arcing which takes place during the initial heating period of each cycle. Aluminum, with a melting point of only 660o C. is not so affected.

It is my belief that aluminum cutlasts the highly heat resistant but hard metals or alloys because it provides a contact of greater area since it is relatively soft and permits any high points or projecting ends of the crystals of the resistor to become embedded in the aluminum and otherwise conforms to the portion of the resistor against which it bears, giving, in effect, a' greater Contact area than would obtain if the end of the resistor were pressed yagainst a hard metal which would not conform to the irregularities of the surface. In the latter case the Contact would be limitedto a comparatively few points. ecause of the greater area of contact, the current densities at the junction are lower and less heat is generated. Moreover, arcing is reduced or eliminated, since the slight embedding of the end portions of the resistor tends to prevent mechanical movement or chafing of the end portion of the resistor against the metal terminal during the initial period of heating.

luminum softens at temperatures considerably below its melting point. It begins to soften at a temperature of approximately 400O C. Its characteristic of softening at only slightly elevated temperatures contributes to the adaptability of aluminum for use as a Contact material because it provides what might be termed a self-compensating contact, inasmuch as the metal will soften at a local hot spot andpermi't the projecting points of the resistor at such a place to penetrate further into the aluminum surface and thereby provide a larger contact surface with a decrease in resistance.

IVhile aluminum melts at approximately 660o C., I have found that it functions satisfactorily as a contact material for resistors operating at approximately 800O to 850o C. The aluminum contact by virtue of its high thermal conductivity reduces the temperature of the surface of the resistor in Contact with it. Hoveover, when the aluminum is heated to about its melting point, it forms a pasty mass with its oxide. Aluminum is therefore quite heat-resistant and can be used where metals of considerably higher melting points melt or burn out.

It is recognized that a point or line contact will carry heavy electric currents if there is sufiicient pressure and the points are capable of conducting heat away rapidly. The one or more points of contact between the resistor element and the metallic terminal are therefore capable of conducting heavy electric currents, but offer a poor path for heat conduction from the heated element to the terminal metal. rIhe terminal metal must also be a good conductor of electricity when its surface is oxidized. Aluminum is a good heat conductor and its oxide is soft and readily penetrated by the contact points under pressure.

In order to more effectively cool the contact points, the aluminum is preferably extended a considerable distance away from the point of engagement with the resistor to provide a materially greater surface from which heat may be dissipated by contact with air and radiation, the remote portions thereof remaining' relatively cold.

The aluminum contacts operate on an entirely different principle from that of the very highly heat-resistant hard metal conrds tacts used with non-metallic resistor rods of the type described. Instead of using a hard highly heat-resistant metal, a metal is employed which is relatively soft, and preferably one which will become softened at a relatively low temperature, so as to permit a penetration and embedding of local high spots of the resistor surface.

Themetal which I prefer and one which is readily available is aluminum. However, other metals having these characteristics may be employed. As described and claimed in the copending application of John A. Boyer, Serial No. 386,998, filed August 19, 1929, certain alloys have been developed having these characteristics. My invention ris not, therefore, limited to a particular metal, but other metals or alloys may be employed within the scope of my claims.

This application is a continuation-in-part of my copending applications Serial No. 740,957, filed Octobern l, 1924; Serial No. 54,193, filed September 3, 1925; Serial No. 7 2,881, filed December 3, 1925; and Serial N o. 386,97 9, filed August 19, 1929.

lVhile I have illustrated and described preferred embodiments of m'y invention, it is to be understood that the invention is not so limited, but may be otherwise embodied and practiced within the scope of the following claims.

vI claim: A

1 In an electric heating device, the combination of a rigid resistor element and terminal members having valuminum contact surfaces in yielding engagement with the resistor element.

2. In an electric heating device, the combination of a rigid resistor element and ter-y minal members having relatively soft metallic contacting surfaces in resilient engagement with the resistor element.

3. In an electric heating device, the combination of a rigid resistor of a crystalline refractory conducting material having a contact-engaging portion, and a contacting member of a soft lowamelting point metal in yielding engagement with said contact-engaging portion.

4. In an electric heating` device, the combination o f a rigid resistor of a crystalline refractory conducting material having contacting portions, and terminal members hiaving aluminum contacting faces having yielding engagementwith said contact portions. 5. An electric heating device comprising a rigid resistor element adapted to become hot in operation, yielding engagement with the opposite ends j of the element, the contact members having a surface metal thereon contacting with-the resistor, which metal is relatively soft at the temperature of operation of the element and which oxidizes onjthe surface at suchtemperand contact members in walls forming a chamber therewithin, sustaining, rod-shaped resistance heating eleature, the softening of the metal enabling the end of the resistor to embed itself therein.

6. An electric heating device comprising a rigid resistor element and contacts in resilient engagement with the ends of the resistorand having a surface metal thereon engaging the resistor which conforms under the pressure at the operating temperature of said heating device to the surface of the resistor.

7. In an electric heating device, the combination of a rigid resistor of a crystalline refractory conducting material having a contact-engaging portion of greater electrical conductivity than the main body of the resister, and a contacting member of a soft low melting point metal in yielding engagement with said contact-engaging portion of the resistor.

8. In an electric heating device, the combination of a rigid resistor having,a terminal contact-engaging portion which operates at a temperature lower than that of the main body of the resistor, and a terminal member having a relatively soft metallic contacting surface in resilient engagement with the terminal contact-engaging portion of the resistor.

9. In an electric heating device, the combination of a silicon carbide resistor having silicon impregnated terminal portions which operate at a temperature lower than that of the main body of thelresistor, and contacting members of aluminum in yielding engagement with said terminal portions of the resistor.

10. In an electric heating device, the combination of a silicon carbide resistance element having a terminal contact-engaging portion of greater electrical conductivity than that of the main body of the resistor element, and a contacting member of a metal which softens at a relativelylow temperature in yielding engagement with said contact-engaging portion of the resistor element.

11. Electrical heating apparatus comprising in combination, a.l self-sustaining, barshaped resistance heating element, and end mounting terminals for said heating element having direct butt-end engagement therewith, one of said terminal mountings comprising a yielding and resilient conductive `said end of the heating'element being yieldingly held in mechanically and electrically operative pCpsition wholly through the resilience Vof sai terminal mounting.

\ 12. In combination in an electrical heating apparatus, 'a supporting frame having sidfe a sel ment having its body portion disposed within said chamber, and provided with terminal 4 portions of substantially the same cross-sectional area as its body portion, and means for yieldingly and detachably mounting said heating element in operative position, said means comprising electrically conductive terminal mounting elements supported in operative position on said frame and With which the extreme ends of said heating element are in direct butt-end engagement, at least one of said terminal mounting elements being resiliently urged into and maintained in forcible mechanically supporting and electrical engagement With the extreme end of a terminal portion of said heating element.

13. Electric heating apparatuscomprising in combination, a self-sustaining, unitary, rod-shaped resistance heating element of an essentially non-metallic composition, said heating element being designed to be operated at temperatures of the order of magnitude of 11000 C., and means for mechanically supporting said heating element in operative position and electrically connecting said heating element in circuit Wholly through pressure exerted endWise of said heating element, said means comprising conductive terminal elements in direct butt-end engagement With terminal portions of said heating element, at least one of said terminal elements being yieldingly urged into said butt-end engagement with the heating element.

14. Electric heating apparatus comprising in combination, a self-sustaining, unitary, rod-shaped resistance heating element of an essentially non-metallic character and of substantially the same cross-sectional area throughout its length, said heat-ing element being designed to operate eiciently at temperatures of the order of magnitude of 1100o C., means for yieldingly supporting said heating element in operative position and electrically connecting said element in circuit, said means comprising conductive terminal elements disposed in direct butt-end engagement With terminal portions of said heating element, at least one of said terminal elements having the contact portion thereof Which is in engagenent vvith'said heating element essentially constituted of an iron chromium alloy faced with a softerrmetal adjacent to the end of the resistor.

15. In a domestic heater a non-metallic unitary self-sustaining resistor element of substantially uniform cross-sectional area, the portions of the resistor adjacent the ends being of higher speciiic conductivity than the body portions of the resistor, and means for supporting said resistor by direct buttend engagement comprising concave recessed members which soften at the temperature of operation and into whose recesses the ends of the resistor project and lit in the course of operation, at least one of said recessed members being resiliently supported, and

harder metal integrally united to the softer recessed metal and shaped to provide said resilient support.

16. The domestic heater described in claim 15 in Which the resistor elements are immediately removable on distortion of the resilient support.

17. The domestic heater described in claim 15 in Which the resistor elements are composed mainly of silicon carbide.

18. In combination in an electrical heating apparatus, a self-sustaining resistance heating element composed mainly of silicon carbide and used at incandescent temperatures, and means for supporting said heating element in operative position, said means comprising an lelectrically conductive element yieldingly urged directly into electrical and mechanical engagement with a terminal portion of said heating element, and exerting its supporting effect through butt-end engagement with said terminal portion of the heating element.

19. In combination in an electrical heating apparatus, a self-sustaining, bar-shaped resistance heating element having integrally formed terminal portions of substantially the same cross-sectional area as the body portion thereof and operated at incandescent temperatures,` and means for resiliently maintaining said heating element in operative position and in electrical connection with a line circuit, said means comprising a yieldable element having a recessed contact portion. Within which said terminal portion tits and With which said terminal portion has direct engagement over its extreme end face, said yieldable element exerting pressure on said heating element in the direction of its longitudinal aXis.

20. In combination in an electrical heating apparatus, a. self-sustaining, bar-shaped resistance element in the form of a molded and heat-hardened composition product of substantially the same cross-sectional area throughout and operated at incandescent temperatures, said resistance element having an integrally formed terminal portion of increased conductivity; and electrically conductive terminal mountings for said heating element, one of said terminal mountings comprising a relatively stiff and yieldable supporting portion and a contact portion of niaterial more highly resistant than said yielding port-ion to deterioration of its electrical Contact conductivity under the operating temperatures and carried by said supporting portion, said heating element being removably heldin operative position through yielding frictional end-Wise direct engagement between said terminal mountings and the eX- treme end of said heating element.

21. In an electrical heating apparatus, a substantially non-metallic molded and selfsustaining electrical resistance heating element having a body portion operated at incandescent temperatures and a terminal portion of substantially the same cross-sectional area of said body portion and of increased electrical conductivity ther-eover, and means for yieldingly maintaining said heating element in mechanically and electrically operatlve position through direct butt-end engagement therewith, said means comprising a terminal mounting including an element of heat-resistant conductive metal, and means for yieldingly urging said latter element into forcible but yielding operative direct engagement with the extreme end of said terminal portion of th heating element.

In testimony whereof, I have hereunto set my hand.

HAROLD N. SHAW. 

